Changing aspect ratio

The first time I changed the aspect ration of my 3840x2160 clip to 2:39 aspect ratio it worked perfectly. I changed it in Master Settings, Timeline resolution. The clip when uploaded to youtube stretched from one end of the browser to another and there were no black bars. (e.x. https://www.youtube.com/watch?v=_nvbYUogCMk)

Ever since then, every time I try to change the aspect ration to 2:39, the clip shrinks. No black bars, just the entire clip shrinks down so that there is black on all sides.

I hate when I get something right and then it doesn't work afterwards. What am I doing wrong, Tero? ;-)

Hey Glenn,
The first time I tried it all I changed was the timeline resolution and it worked. Every time since I've tried everything, changed the video format from UHD 2160p to 4K 2160p, changed resolution on the delivery page to match (2:39:1) but it just makes the clip smaller on all sides.

Now that I re-read your response that sounds like something I did the first time. Let me try again.
*Yep, that did it. Thanks, Glenn!